1. Field of the Invention
The present invention described in the appended claims of this patent application relates to an echo canceller which operates in such a circumstance that a sound wave released by a signal/sound transducing means (e.g., speaker) happens to be picked up by a sound/signal transducing means (e.g., microphone) and works to suppress an echo signal in an audio signal produced by the sound/signal transducing means, and also relates to a line echo canceller which eliminates an electrical echo arising on a communication line.
2. Description of Related Art
In a telephone conference system or television conference system for exchanging vocal information among user terminals, a speaker as signal/sound transducing means and a microphone as sound/signal transducing means of each terminal are placed to have a sort of acoustic coupling with each other. Therefore, a sound wave released by the speaker is picked up by the microphone unintentionally.
A sound wave released by a speaker and picked up by a microphone becomes an adverse echo signal in an audio signal produced by the microphone. If the audio signal produced by the microphone includes such an echo signal, a resulting echo-added sound wave reproduced from this audio signal and released by a speaker of other user terminal will have a degraded sound quality in the performance of vocal information exchange for example.
To cope with the matter that a sound wave released by a speaker is picked up by a microphone unintentionally, there has been proposed the provision of a pseudo echo signal generator which produces a pseudo echo signal based on the audio signal to be fed to the speaker so that the echo signal included in the audio signal produced by the microphone is cancelled out by the pseudo echo signal produced by the pseudo echo signal generator. (Refer to the following non-patent publication #1, non-patent publication #2, and patent publication #1, for example).
In a conventional vocal information exchange system with the ability of echo suppression, as shown for example in FIG. 7, a microphone 11 and a speaker 12 are so located that a sound wave released by the speaker 12 happens to be picked up by the microphone 11 (indicated by the dashed-line arrow), and an incoming audio signal Ss which is introduced through a receiver 13 is fed to the speaker 12 by being amplified by an audio signal amplifier 14 and also put in to a pseudo echo signal generator 15. The speaker 12 releases a sound wave which is reproduced from the incoming audio signal Ss.
The microphone 11 produces an audio signal Sm from a sound wave sensed by it, and the audio signal Sm includes an echo signal which is derived from a sound wave released by the speaker 12. The audio signal Sm is amplified by an audio signal amplifier 16 and put in to a subtracter 17.
The subtracter 17, which has another input of a pseudo echo signal Sei produced by the pseudo echo signal generator 15, subtracts the pseudo echo signal Sei from the audio signal Sm produced by the microphone 11, thereby suppressing the echo signal included in the audio signal Sm, and puts out a resulting subtracted audio signal Sma. The subtracted audio signal Sma is put in to a subsidiary echo suppressor 18 and also to the pseudo echo signal generator 15. Accordingly, the pseudo echo signal generator 15 produces the pseudo echo signal Sei based on both the incoming audio signal Ss to be fed to the speaker 12 and the subtracted audio signal Sma put out from the subtracter 17.
The subsidiary echo suppressor 18 further suppresses a residual echo signal included in the subtracted audio signal Sma put out from the subtracter 17 thereby to produce an outgoing audio signal Sma′. The outgoing audio signal Sma′ is sent out by a transmitter 19.
The pseudo echo signal generator 15 is an adaptive filter which is arranged as shown in FIG. 8 for example. The adaptive filter is made up of: a delay circuit including unit delay elements of n in number that is larger than two 20(0), 20(1), 20(2), . . . , 20(n−2) and 20(n−1) which are connected in series to delay in steps the incoming audio signal Ss to be fed to the speaker 12; a factor generator 21 which produces factor signals h(0), h(1), h(2), . . . , h(n−2) and h(n−1) of n in number, which vary in response to the subtracted audio signal Sma put out from the subtracter 17; factor registers of n in number 22(0), 22(1), 22(2), . . . , 22(n−2) and 22(n−1) which hold the respective factor signals; multipliers of n in number 23(0), 23(1), 23(2), . . . , 23(n−2) and 23(n−1) which implement multiplication between delayed audio signals x(0), x(1), x(2), . . . , x(n−2) and x(n−1) put out from the respective unit delay elements and the factor signals h(0), h(1), h(2), . . . , h(n−2) and h(n−1) put out from the respective factor registers correspondingly; and a summing operator including adders of n−1 in number 24(1), 24(2), . . . , 24(n−2) and24(n−1) which sum the multiplication outputs y(0), y(1), y(2), . . . , y(n−2) and y(n−1) of the respective multipliers cumulatively.
The adders 24(1)-24(n−1) of the summing operator produce summation outputs z(1), z(2), . . . , z(n−2) and z(n−1), respectively, and the output z(n−1) of the last-stage adder 24(n−1) is released as the pseudo echo signal Sei. The pseudo echo signal Sei, i.e., summation output z(n−1), is the total of the multiplication outputs y(0)-y(n−1).
The factor generator 21 implements a learning-based revision process for the factor signals h(0)-h(n−1) based on the NLMS (Normarized Least Mean Square) algorithm for example. Specifically, the factor signals h(0)-h(n−1) are updated in accordance with the following formula (1).h(i)(t)=h(i)(t−1)+μ·e·x(i)/X (i=0, 1, 2 . . . , n−1)  (1)
In the formula, h(i)(t) represents a factor signal which is held at time t by the i-th factor register 22(i), h(i)(t−1) represents a factor signal which is held at time t−1 by the i-th factor register 22(i), μ is a revision factor which is larger than 0 and smaller than 2, e is the subtracted audio signal Sma, x(i) is a delayed audio signal put out from the i-th unit delay element 20(i), and x is the square-sum of the delayed audio signals x(0)-x(n−1).
In this vocal information exchange system with the ability of echo suppression, the subsidiary echo suppressor 18, which further suppresses a residual echo signal included in the subtracted audio signal Sma put out from the subtracter 17 thereby to produce an outgoing audio signal Sma′, is a nonlinear processing means as described in the following non-patent publication #2 for example, which is designed to implement a nonlinear-wise level control process for the subtracted audio signal Sma thereby to alleviate the echo. It is necessary for this processing means to control the level of subtracted audio signal Sma accurately so that the vocal information signal carried by the subtracted audio signal Sma is not attenuated for the avoidance of the deterioration of quality of the sound wave which will be reproduced from the outgoing audio signal Sma′ put out from the subsidiary echo suppressor 18.
A specific scheme of controlling the level of subtracted audio signal Sma by the nonlinear processing means as the subsidiary echo suppressor 18 for minimizing the deterioration of quality of the sound wave reproduced from the audio signal Sma′ is to base the control on the degree of echo suppression by the pseudo echo signal Sei provided by the adaptive filter as the pseudo echo signal generator 15.
The conventional system employs an echo suppression degree calculator 25 as shown in FIG. 9 for assessing the degree of echo suppression achieved by use of the pseudo echo signal Sei produced by the adaptive filter as the pseudo echo signal generator 15. This echo suppression degree calculator 25 is made up of: a subtracted signal power calculator 26 which calculates power of the subtracted audio signal Sma put out from the subtracter 17 and produces a subtracted power signal Pma; an incoming signal power calculator 27 which calculates power of the incoming audio signal Ss to be fed to the speaker 12 and produces an incoming power signal Ps; and a division operator 28 which has inputs of the subtracted power signal Pma from the subtracted signal power calculator 26 and the incoming power signal Ps from the incoming signal power calculator 27.
The division operator 28 divides power of subtracted audio signal Sma indicated by the subtracted audio signal Sma with power of incoming audio signal Ss indicated by the incoming power signal Ps, thereby producing an echo suppression degree signal Ses indicative of the degree of echo suppression achieved by the adaptive filter as the pseudo echo signal generator 15. Namely, the degree of echo suppression by the pseudo echo signal Sei and indicated by the echo suppression degree signal Ses is derived from power of subtracted audio signal Sma divided by power of incoming audio signal Ss.
Non-patent publication #1: “Adaptive Signal Processing” written by B. Widrow and S. D. Stearns, published by Prentice-Hall in 1985.
Non-patent publication #2: ITU-T (TELECOMMUNICATION STANDARDIZATION SECTOR OF INTERNATIONAL TELECOMMUNICATION UNION) Recommendation G.165 (03/93), ECHO CANCELLERS.
Patent publication #1: Japanese Patent Unexamined Publication No. 56526/1986.